1. Field of the Invention
The present invention relates to a rewritable recording medium, an image recording set, and an image recording method for writing by an image recording device to be externally driven.
2. Description of the Related Art
In recent years, electronic paper that has visibility and portability close to that of a printed material, does not require a backlight, and is low in power consumption, thin, and lightweight, has been put into practical use and gained attention. Electronic paper is classified into internally-driven paper-like display (hereinafter referred to as PLD) that includes a drive element inside and can display images by itself, and externally-driven rewritable paper using a recording medium on which writing is performed by a printer. The rewritable paper includes all recording media such as paper and sheets on which rewriting can be repeatedly performed, and is synonymous with a rewritable recording medium in the present invention.
Various types of electronic paper have been proposed, and among them, as a PLD, an electrophoretic type that performs display by changing an optical reflectance by causing white and/or colored particles charged in a liquid to migrate to the surface side or the opposite direction of a display medium by an electric field is known as a most practical type. For example, a display or recording device is disclosed which is configured so that by applying an electric field to a mixed multi particle dispersion system including particles that migrate in directions opposite to each other due to an electric field, certain particles are optically shielded to change the optical reflection characteristics of the dispersion system (see Japanese Patent Application Publication (JP-B) No. 50-15115).
In addition, an electrophoretic display device is disclosed in which a large number of microcapsules in which a dispersion system obtained by dispersing electrophoretic particles in a colored dispersion medium is encapsulated are provided between a pair of counter electrode plates (see Japanese Patent (JP-B) No. 2551783).
Further, an ink is disclosed in which by applying an electric field to microcapsules including first particles and second particles with polarity reverse to the polarity of the first particles, either the first particles or the second particles are caused to migrate to the surface (see JP-B No. 4410135). Thus, the electrophoretic type, particularly, a microcapsule type electrophoretic type has already been put into practical use as electronic paper, and has gained much attention.
These electrophoretic electronic papers can retain displayed content without requiring electric power after displaying the content once, and this is recognized as one of the advantages, however, there are problems in image retainability in actuality because if an image is left without application of an electric field after it is displayed, the image density changes with time, and if the image is influenced by static electricity and pressure, from the outside, the image quality is deteriorated.
To solve these problems, for example, a method for controlling a solubility parameter in a dispersion medium by making walls of microcapsules from a graft copolymer (see Japanese Patent Application Laid-Open (JP-A) No. 2007-286124) and a method for dissolving or dispersing a polymer that does not increase viscosity in a suspending fluid (see JP-A No. 2010-002933), are disclosed.
These technologies are effective for improving the image retainability required for PLD. However, these are not sufficient for realizing image retainability required for rewritable paper.
The reason for this is differences in structure and purpose of use between the PLD and the rewritable paper. The PLD is provided with a substrate having conductive layers as electrodes above and below a display layer that is a region in which electrophoretic particles move and covered by a housing, so that it is hardly influenced from the outside, and a voltage can be applied thereto as appropriate, and accordingly, the PLD can retain an image.
However, the rewritable paper is not covered by a housing and is easily influenced from the outside, and in addition, a recording medium does not include an element or power source for applying a voltage, so that it is impossible to retain an image for a long period of time from the outside. Although the influence from the outside can be reduced by a covering by a protective layer, in some cases, this is not sufficient as an effect of enhancing image retainability, and the advantages such as flexibility and low cost required as a rewritable paper are lost.
Rewriting frequency of the PLD is high in use, so that the extent of demand for image retainability is not so high, however, the rewritable paper is highly demanded to retain records for a long period of time like paper, so that semipermanent image retainability must be realized.
As a technology for significantly enhancing image retainability, a number of methods for fixing colored particles have been disclosed in which the viscosity of a dispersion medium in which electrophoretic particles are dispersed is controlled by temperature, and after recording is performed by reducing the viscosity by heating, cooling is applied to fix colored particles.
For example, the disclosed methods are a method using resin, rubber, solder, or synthetic wax that are solids at normal temperatures and are softened by heating or a solvent (see JP-B No. 50-15115), a method in which two phases of liquid and solid coexist in a dispersion liquid containing electrophoretic particles, a dispersion medium, a dye, and a dispersant when the dispersion medium is at a normal temperature, and when the electrophoretic particles electrophoretically migrate, the solid phase is dissolved and the two phases are converted into a uniform liquid phase (see JP-B No. 4168556), a method in which a heat-meltable substance that is solid when it is not heated and becomes liquid when it is heated, specifically, waxes, saturated fatty acid, higher alcohols, and electrophoretic fine particles are encapsulated into microcapsules (see JP-A No. 2001-301325), a method using a mixture that has fluidity at an electrophoretic operating temperature, and becomes an optically opaque solid at a temperature lower than the electrophoretic operating temperature, specifically, a mixture of a substance selected among long-chain alcohols, phenols, aromatic ketones, decanes, and dodecanes, and a substance selected among paraffin, solders, waxes, and saturated hydrocarbons (see JP-B No. 4114374), a method using higher paraffin hydrocarbons, solders, waxes, or aromatic hydrocarbons as a dispersion medium that is solid at normal temperatures and becomes liquid by heating (see JP-A No. 2003-91022), and a method in which a dispersion medium is composed mostly of rice wax and contains a surfactant (see JP-A No. 2007-140367).
Thus, a dispersion medium that is solid at normal temperatures and becomes liquid when being heated is very effective for improvement in image retainability because electrophoretic particles are fixed by cooling after recording.
However, in these technologies, waxes, solders, and saturated fatty acid, are added to the dispersion medium, so that the temperature responsiveness in transition between the solid state and the liquid state is low, the rewriting speed decreases, and the viscosity of the dispersion medium is not sufficiently lowered even by heating, the contrast deteriorates, and image quality deteriorates due to repeated rewriting, so that satisfactory performance is not obtained.
A method using a dispersion medium that turns into a gel in a PLD is disclosed. For example, a thermal writing system is disclosed which includes a dispersion medium that turns into a gel and a sol (liquid) reversibly according to temperature and electrophoretic particles, and locally causes the electrophoretic particles to migrate by locally converting the dispersion medium into a sol (see JP-A No. 02-196227).
However, with this technology, as a dispersion medium that turns into a gel and a liquid, specifically, a high-molecular substance whose solvent becomes a poor solvent is used, however, the high-molecular substance is dissolved when the temperature of the solvent is high, and the high-molecular substance becomes insoluble and turns into a gel when the temperature is low, so that the high-molecular substance itself does not have thermal reversibility.
Therefore, the temperature responsiveness in transition between a gel and a liquid is low, and liquefaction is local, so that the display speed and image quality are not satisfactory.
In addition, a display gel for electrophoretic display including a dispersion medium, white particles, colored particles, and a gelling agent has been disclosed (see JP-A No. 2003-149691).
However, in this technology, although a gelling agent is contained in the dispersion medium, thermal reversibility is not described, and it is described that, due to gelatinization, the dispersion medium is substantially equally divided into continuous and/or discontinuous minute regions and turns into a gel, and the particles electrophoretically migrate by the action of an electric field in the dispersion medium in a gel state. Therefore, the problems in the rewriting speed and contrast are not solved, and a solution to the problems is still not attained.
On the other hand, a display medium for electrophoretic display containing a liquid system, a set of a thermoreversible gelling agent and colored particles in the PLD has been disclosed (see JP-A No. 2007-11342). With this technology, by containing the thermoreversible gelling agent, the display medium turns into non-gel and is reduced in viscosity at a temperature close to or higher than a melting point of the gelling agent, so that images can be formed in a display device.
When the display device is cooled after an image is formed, the medium turns into a gel, so that even without applying an electric field, the image is stably maintained. In the method using a thermoreversible gelling agent, the gelling agent itself causes gelatinization, so that the temperature responsiveness is higher and more stable than in the conventional technologies in which waxes and solders are added or gelatinization is controlled by solubility of a high-molecular substance in a solvent, so that this method is very excellent for enhancing image retainability.
However, as long as the thermoreversible gelling agent is also controlled by temperature, the influence of reduction in rewriting speed on use with frequent image rewriting increases. As described above, this technology using a thermoreversible gelling agent is premised on that it is used for a display device having two parallel transparent conductor electrode panels, that is, a PLD, so that the disadvantage of deterioration in display responsiveness becomes greater than the advantage of semipermanent image retainability by using a thermoreversible gelling agent. In addition, a heating unit and a cooling unit must be provided inside the device, so that the thickness and weight of the PLD increase, and this is also a big disadvantage.
Therefore, the effect of use of the thermoreversible gelling agent has not been sufficiently utilized, and has not been applied to a PLD in actuality.
The PLD mainly used as a conventional display has currently been largely developed and significantly advanced, however, the advancement of the rewritable paper mainly used as a conventional paper has been slightly delayed as compared with the PLD.
However, there is a need for paper different from that for a display, and the spread of rewritable paper that can be handled in the same manner as paper has been expected.
Problems of rewritable paper exist in image retainability, image quality, cost, safety, flexibility, and toughness (thinning, reduction in weight).
It is considered that the need for image retainability required for rewritable paper is higher than that for PLD as described above. In order to perform rewriting, the PLD is configured as a device, so that the rewriting frequency is naturally high. Therefore, it does not necessarily require semipermanent image retainability. On the other hand, the rewritable paper is rewritable, however, on the assumption that it is used instead of paper, high image retainability is required. In addition, rewritable paper that is not covered by a housing is more easily influenced by static electricity and pressures from the outside than the PLD covered by a housing, so that this is a big problem in enhancing image retainability.
Therefore, image retainability is a very important issue for rewritable paper rather than for PLD.
For image quality required for rewritable paper, in particular, white and black contrast ratio is important. A printed material has a very high contrast ratio, so that its visibility is high, however, electronic paper is insufficient particularly in white reflectance, and the reality is that the contrast of the electronic paper lags behind that of a printed material. One of the possible reasons for this is the influence of microcapsules enveloping electrophoretic particles and a dispersion medium. The used microcapsules have high transparency, however, it cannot be denied that microcapsules are the cause of deterioration of the reflectance as long as the microcapsules cover electrophoretic particles. In addition, if the microcapsules are reduced in diameter, the regions of capsule walls in which recording cannot be performed increases, so that the image quality becomes much lower. Microencapsulation of electrophoretic particles and a dispersion medium enables these to be handled as electronic ink, however, in terms of image quality, it is better that microcapsules are not used.
The cost required for the rewritable paper is one of the biggest issues that hinder its spread. The PLD is mainly used as a conventional display, so that an increase in cost is allowed to some extent, however, rewritable paper is mainly used as conventional paper, so that high cost is a fatal issue. It is assumed that due to the rewritability, the unit cost per one rewritable paper becomes higher than the unit cost per one paper, however, a value of the rewritable paper, different from the value of a display, can be found in its capability of mass-recording and capability of being distributed like paper, so that if the unit cost excessively increases, the value of the rewritable paper becomes lower.
For electrophoretic type PLD, microencapsulation technology is relatively advantageous in cost as compared with conventional methods, however, for rewritable paper that replaces paper, the cost of microencapsulation technology is not on a practical level. Further, a configuration including lamination of a conductive layer and a protection layer as electrodes in addition to a recording layer in which electrophoretic particles are dispersed costs almost as much as a simple display, so that the value as rewritable paper cannot be found.
An important point in safety required for rewritable paper is prevention of leakage of organic solvents. Organic solvents are used in electrophoretic type and/or magnetophoretic type electronic paper. The risk of leakage and contact of organic solvents with humans is low even when the electronic paper is broken and if it is covered by a housing like PLD, however, in the case of rewritable paper, the risk of leakage and contact of organic solvents with humans is high when the rewritable paper is broken because it is not covered by a housing.
Therefore, in order to realize electrophoretic type and/or magnetophoretic type rewritable paper, a safety measure for preventing humans from coming into contact with organic solvents and inhaling the organic solvents is essential. In addition, microcapsules may contain formaldehyde, and this is also a big issue from the viewpoint of safety.
Concerning flexibility required for the rewritable paper, the PLD has also been developed to be made flexible, and for rewritable paper that has been demanded as a replacement for paper, flexibility is a necessary characteristic. To make the rewritable paper flexible, the recording medium itself must be a lightweight thin film, and flexibility is also important for enhancing toughness. To spread the rewritable paper as a replacement for paper, it is very important to add a value different from the value of PLD by making the rewritable paper thinner or lighter in weight like paper and increasing flexibility and toughness.
As described above, as an externally-driven rewritable recording medium on which writing is performed by a printer, provision of an externally-driven rewritable recording medium that includes image retainability, image quality, low cost, safety, and flexibility has been demanded.